The present invention relates to articles having a temperature indicator applied thereon. The temperature indicator contains a bonding agent and at least one fine-grained, organic substance that is largely opaque below a defined temperatue and melts when the defined temperature is reached and thus changes its optical properties, so that the temperature indicator becomes transparent or translucent.
Temperature indicators, or thermochromic coatings are materials that change color at a certain temperature. Thermochromic materials have been used as temperature indicators in heat-shrinkable products such as, for example, heat-shrinkable collars of cross-linked polymers. To this end, in such heat-shrinkable products, heat-activatable sealants, for example hot-melted glues, are applied onto an inside surface. The sealants melt at a defined temperature, producing a desired, sealing connection between the heat-shrunk product and the substrate to be enveloped.
The necessary heating to activate the material is provided through the use of an external heating source, for example a flame. The heat activation, however, can require greater heating than is required for the actual shrink process of the heat-shrinkable product. In order to ensure that the necessary heat activation has occurred on the inside, the outside surface of the heat-shrinkable product is provided with a temperature indicator. The temperature indicator, by changing color, indicates that the temperature at which one can assume that the required heat activation of the sealant has been successfully carried out at the inside, has been reached on the outside of the surface.
A temperature indicator can also be used to avoid subjecting the outside of the material to unnecessary heating that can damage the material under certain circumstances. The temperature indicator must also be irreversible, so that it is guaranteed that the color change, once it has occurred during the thermal treatment, remains.
U.S. Pat. No. 4,344,909, for example, discloses thermochromic compositions that cerntain an organic material that decomposes while producing a visible color change. This color change proceeds, for example, from green to black, whereby a certain coking occurs among other things. The decomposition products of the organic material are colorless and pigments are added to achieve the initial green color. Gaseous decomposition products, as well as carbon-like and tar-like residues on the surface of the collar are produced in these decomposition reactions. The formation of craters can occur. Moreover, there is the risk that the original color will be restored due to the reaction of the soil moisture as a consequence of the release of undecomposed, green pigment particles.
European Patent 0 042 285 discloses a countermeasure thereto. A bonding agent is provided that enters into an interaction with the decomposed organic material in order to prevent a leaching process from the composition.
Initially, inorganic pigments that experience a change in color at a prescribed temperature were often used for temperature indication. Salts of heavy metals such as lead, cobalt, cadmium, nickel, chromium, mercury or copper were predominantly used as such pigments (see Chemie-Lexikon, Dr. Hermann Roempp, Second Edition, 1950, page 1631). For example, such compounds were mixed with a suitable bonding agent composed of a synthetic resin and spirits and were then applied onto the articles to be monitored with brushes or, respectively, spray guns.
A disadvantage of the use of such materials is that they can result in environmental pollution. Under certain circumstances, moreover, inorganic thermochromes damage the shrinkable material. This is particularly true during the shrink process. Additionally, craters can form on the surface of the material, whereby a catalytic decomposition reaction on the surface can also be anticipated; this still being capable of acting even during later use. U.S. Pat. No. 4,121,011 discloses the addition of additives to such compounds to alleviate this later disadvantage.
Another attempt at solving the problems, that have been encountered with temperature indications, is to use materials that are essentially opaque initially condition, but, which melt when a predefined temperature is exceeded and thereby change their optical properties and become transparent or, respectively, translucent. This causes the substrate or, resepctively, the color of the substrate to become visible allowing this event to be utilized as a temperature indication.
For example, German Published Application 33 07 567 discloses a heat-restroable article whereat an outer layer of polyethylene or polypropylene that is opaque at normal temperature is extruded on. When the crystallization temperature is reached, the outer, continuous layer becomes transparent and the color of the inside layer becomes visible.
German Published Application 33 26 021 discloses the use of a coating composition composed of a pigment and of a bonding agent as a temperature indicator in a heat-restorable article. The pigment and bonding agent are composed of materials having different refractive indices. When the melting temperature of the pigment and/or bonding agent is reached, a substances having a uniform refractive index is formed. This change in the refractive index leads to a visible change in color.
U.S. Pat. No. 2,269,038 discloses color indications wherein fine-grained substances are employed with a suitable bonding agent. These substances are applied as a film on non-modifiable foundations. When a temperature prescribed by the material is exceeded, this substance composed of the inorganic pigment and of the bonding agent melts. The result of the process is that the coat becomes transparent, allowing the color of the foundation to be seen.
U.S. Pat. No. 2,928,791 discloses a temperature indicator composed of a white, crystalline, organic substance that melts when the melting temperature is reached and is absorbed by the carrier. A visible, irreversible color change thereby results. Here, too, color indicators are applied on rigid, invariable foundations.
U.S. Pat. No. 3,002,385 also discloses organic substances and mixtures of inorganic salts for temperature indication. When a given temperature is reached, the material melts and is absorbed by the colored foundation.